Many control components in plants, buildings, and other manufacturing facilities utilize hydraulic pressure to position the actuators, control valves, or operating surfaces of these components. Such components include steam control valves, fuel valves, dampers, vanes, etc. One common means of positioning the actuators is to provide a linearly increasing variable hydraulic pressure that acts upon the piston of a linear hydraulic actuator or vane of a rotary cylinder. The opposing force required to counterbalance this variable pressure and thus create proportionality can be in the form of an opposing spring, or hydraulic pressure.
While purely hydraulic valving and control systems have been utilized to effectuate the positioning of these control components, modern electronic controls have increased the functionality and flexibility of the system control. Such component, system, and plant controllers typically utilize PLC- or DCS based computing systems to monitor and control the various components within the system. The use of such controllers, therefore, necessitates the use of an interface component that is capable of taking the control signal outputs from such controllers and converting those electronic control signals into hydraulic control signals that can effectuate the positioning and control of the hydraulic actuated components. One such interface control device is known as a current to pressure converter (CPC).
A typical CPC is configured to receive an analog 4-20 mA control signal from a system or plant controller. This 4-20 mA control signal is then proportionally converted into a hydraulic output pressure by the CPC. As such, the CPC may be thought of as a electrohydraulic, pressure regulating valve. Such CPCs typically include an internal 3-way valve, actuator, pressure sensor or pressure feedback mechanism, and on-board analog electronics. A cascade control loop is typically employed to achieve closed loop control of pressure. The first control loop compares the input control signal or pressure setpoint to the measured feedback. The difference is then modified by a circuit or algorithm to generate a position demand signal which is the input of the second control loop. The position demand signal is then compared to the measured position and the difference modified by a circuit or control algorithm to produce a drive signal which will open or close the actuator to match the position demand over time. The combined operation of the dual control loops in conjunction with the actuator and valve ensures that the measured feedback matches the setpoint over time.
The valve internal to the CPC is a three way control valve. At the center position, the control port is isolated from both the supply and drain. By moving the valve slightly above the center position, the control port is connected to the supply port resulting in an increase in pressure. By moving the valve below the center position, the control port is connected to the drain, resulting in a decrease in pressure. A return spring is provided in the assembly such that in the event of loss of power or an electric fault, the valve will move to the “minimum pressure” position which in most applications is the direction to shut down the turbine.
While current CPC's perform adequately in many applications, the accuracy of such control in some installations may be adversely affected by the thermal drift associated with the analog control circuits within the CPC itself. Further, CPC malfunction has been noted in some systems that do not typically change the positioning of the control component for long periods of time, or in backup CPC's in systems that utilize a primary and backup regulator to ensure system operation in case of malfunction of the primary CPC. Such malfunctions have been determined to be caused by the build-up of silt and other contaminates that have accumulated on the valve element during a long period of stagnant control.
In view of the above, the inventor has recognized a need for a new and improved CPC that overcomes the inaccuracies resulting from thermal drift of the analog control circuits and that ensures continued operation even after extended periods of inactivity that would otherwise result in silt build-up on the valving element leading to malfunction. Embodiments of the present invention provide such a new and improved CPC.